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EE689 Lecture 12


Title: EE689 Lecture 12 Author: Steven Woodward Last modified by: Steven Woodward Created Date: 2/24/1999 2:34:08 PM Document presentation format – PowerPoint PPT presentation

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Title: EE689 Lecture 12

EE689 Lecture 12
  • Review of last lecture
  • Multicast basics

Multicast -motivation
  • Point-to-point delivery not efficient for
    events/transmissions of general interest
  • Examples
  • News multicast
  • IETF sessions/rock concerts
  • Many receivers may share the same path
  • Point-to-point delivery too expensive

Multicast motivation
Multicast issues
  • In point-to-point delivery, receiver address is
    known gt routing is straightforward
  • In Multicast, many receivers
  • How to transmit data to all the receivers?
  • How to identify the group of receivers?
  • At the sender?
  • In the network?

Multicast issues
  • Identify multicast by a group/multicast address
  • The membership changes as the receivers
    join/leave the group
  • Routers/Network need to recognize the multicast
  • Receivers need to receive on their own IP address
    and on the multicast address

Multicast routing
  • For point-to-point delivery, a router looks up
    routing table and knows how to forward
  • For multicast, many receivers
  • may have to forward packets on multiple outgoing
  • too hard to keep track of individual receivers at
    each router for each multicast group
  • remember just the links on which to be forwarded
    - change as receivers join/leave

Multicast addressing
  • A multicast sender uses the group address as the
    receivers address when sending packets
  • Network/routers keep track of actual receiving
    subnets/paths for this group address (not the
    actual receivers)
  • Receivers can reply to senders address or to
    group address

Multicast addressing
  • Part of IP address space reserved for multicast
  • Multicast routers recognize multicast addresses
  • Need to get a multicast address for a
    transmission before multicast can start
  • Protocols exist for obtaining multicast addresses
    and finding out a multicast address

Class D addresses
  • High order 4 bits 1110, followed by a 28-bit
    multicast group ID. -
  • - all systems on this subnet
  • - all routers on this subnet
  • - all DVMRP routers
  • - all OSPF routers

  • The end routers use physical layer multicasting
    to multicast packets to multiple receivers on the
    same subnet
  • IP-multicast group ID copied into the MAC-layer
    multicast address
  • All receivers of this group listen to this
    multicast address to receive packets

  • Internet Group Membership Protocol
  • Used to join a multicast group and to check on
    the current status of receivers on a subnet
  • IGMP -join message propagated up the routers
    until the multicast tree reached.
  • Routers periodically poll hosts on subnets to see
    if any active receivers remain

  • If no active receivers remain, routers propagate
    leave messages upstream to reduce unnecessary
  • Frequent polling can increase overhead
  • Separate protocols for finding group membership
    address - sd

IGMP version 3
  • Allows sender-specific reports when multiple
    senders are in a multicast group
  • Can exclude certain senders and allow certain
    senders - allows better bandwidth management in
    multi-sender multicasts.

  • Multicast Backbone
  • Consists of all the multicast-enabled routers
  • If two multicast routers are not directly
    connected, uses tunneling over non-multicast
  • Allows gradual deployment

Multicast routing
  • Need to recognize multicast addresses
  • The routing tables change as the receivers
    join/leave a multicast group
  • Every router keeps track of downstream links on
    which to forward a packet
  • Routing table and multicast address expire at
    the end of session

Multicast routing
  • Many possible approaches
  • Flooding
  • send on all links to reach the receivers
  • not efficient
  • Spanning tree
  • efficient
  • could concentrate traffic on a few links

  • Spanning trees rooted at the sender
  • Sender sends out a broadcast over the entire
    network - all routers get at least one packet
  • When receivers want to join, routers employ
    Reverse Path Multicasting
  • Use Pruning to limit the multicast transmission.

  • DVMRP - Distance Vector Multicast
  • MOSPF - Multicast Extensions to OSPF
  • PIM - Protocol Independent Multicast

  • Spanning trees rooted at the sender
  • When a receiver wants to join a group, may have
    to broadcast on all upstream links to find a path
    to the sender
  • could cause a lot of overhead in sparse groups
  • need better solutions

Sparse Mode routing
  • Use a Core-based tree (CBT)
  • Use a rendezvous point or a core router
  • Direct all joins to RP which knows how to reach
    the sender
  • can avoid broadcasting multicast group
    information to all routers in the network
  • can result in non-optimal paths
  • would need to modify multicast tree over time